1. Field of the Invention
This invention relates to a chuck used to hold a substrate during a lithographic process. More specifically, this invention relates to a chuck capable of flexing during a lithographic process thereby altering the surface topography of a substrate mounted on the chuck.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the relevant art would recognize that other substrates could be used without departing from the scope of the instant invention.
During lithography, a wafer is disposed on a wafer stage and held in place by a chuck. The chuck is typically a vacuum chuck capable of securely holding the wafer in place. The wafer is exposed to an image projected onto its surface by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the relevant art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the sharpness of the image projected onto the wafer is maximized. While using a step-and-scan technique generally assists in improving overall image sharpness, image distortions generally occur in such systems due to imperfections within the projection optics system, illumination system, and the particular reticle being used.
One technique for improving image sharpness has been proposed by Stagaman (U.S. Pat. No. 5,563,684). Stagaman observes that a conventional approach for improving image sharpness is to use a deformable chuck that flattens the wafer surface in order to conform that surface to the focal plane of the lens used. However, Stagaman further observes that the actual image pattern associated with a lens can differ from the theoretical flat focal plane of the lens, and so flattening the wafer""s surface will not necessarily improve pattern sharpness for a particular lens. Thus, Stagaman suggests an approach whereby the actual focal pattern of a lens is determined. A deformable chuck is then used to conform the surface of the wafer to the actual focal pattern of the lens, thereby improving average image sharpness. Stagaman""s chuck appears to work with individual adjustable pins that are extended or retracted in order to bend the surface of the wafer. The operation and details of these individually extendable pins are not fully described by Stagaman.
Taniguchi et al. (U.S. Pat. No.4,475,223; xe2x80x9cTaniguchixe2x80x9d) appear to describe another type of flexible chuck that uses individual displacement units to produce displacement of a wafer disposed on the chuck. Such displacement units work individually, like the pins of Stagaman""s chuck, discussed above. These displacement units can be screw elements controlled by DC motors. Taniguchi also describes that the displacement units can be various type of deformable elements, including piezoelectric elements.
Finally, MacDonald et al. (U.S. Pat. No. 5,094,536; xe2x80x9cMacDonaldxe2x80x9d) appear to describe another type of flexible chuck that uses individual actuators with extendable pins. The pins are arranged in an array on the chuck, the tips of the pins providing the surface on which a wafer can be placed. MacDonald""s pins are formed of individual piezoelectric crystals. During operation, an electrical signal is supplied to each of the crystals, causing deformation of the wafer provided atop the array of pins.
In each of the approaches discussed above, actuators are individually formed and provided at the chuck surface. It is thus difficult to dispose the actuators such that each actuator surface lies in a single plane when the actuators are in an unbiased state. Furthermore, it is difficult to produce a high density of actuators according to the conventional structures discussed above.
Thus, what is needed is a simple to manufacture flexible chuck having a high density of actuators with coplanar contact surfaces.
The instant invention provides a flexible chuck that overcomes the shortcomings of the conventional flexible chucks discussed above.
In one embodiment, a flexible chuck for supporting a substrate during lithographic processing is described. This flexible chuck includes an electrode layer, a piezoelectric layer disposed on the electrode layer, and a substrate support layer disposed above the piezoelectric layer. The piezoelectric layer deforms in response to voltage variations applied across the layer. Thus, by providing electrical signals to the piezoelectric layer through the electrode layer, the substrate support layer can be flexed, thereby changing surface topography on a substrate disposed on the flexible chuck. The magnitude of the electrical signals provided affects the amount of deformation within the piezoelectric layer, and thus the degree of flex exhibited by the substrate support layer.
The substrate support layer can include projections, each of the projections corresponding to a respective electrode within the electrode layer. Furthermore, the substrate support layer can be formed of a conductive material and thus serve as a ground layer. Alternatively, separate substrate support and ground layers can be provided. The flexible chuck in accordance with the instant invention can be a vacuum chuck.
A piezoelectric layer within a flexible chuck in accordance with the instant invention can include projections corresponding the projections within the substrate support layer. Alternatively, the piezoelectric layer can be substantially flat. Alternatively, rather than a piezoelectric layer, a flexible chuck in accordance with the instant invention can include a plurality of discrete piezoelectric elements formed from a substantially flat piezoelectric layer disposed on the electrode layer.
Projections within a flexible chuck in accordance with the instant invention can include one of point projections and strip projections. Additionally, a solid circular projection region can be included that corresponds to a peripheral top surface of the flexible chuck.
A flexible chuck in accordance with the instant invention can further include power and signal interconnection lines for addressing individual electrodes within the electrode layer.
A flexible chuck in accordance with the instant invention can also further include at least one grounded guard ring for confining electric fields produced by the electrical signals applied to the piezoelectric layer.
A flexible chuck in accordance with the instant invention can also further include an interconnection back plate that incorporates at least one of multiplexing and switching hardware.
A flexible chuck in accordance with the instant invention can also include a cooling plate.
A flexible chuck in accordance with the instant invention can also include both a main chuck assembly and a riser assembly for lifting a substrate off of the substrate support layer.
A method of monitoring topographic changes within a flexible chuck in accordance with the instant invention is also desribed. The method can include a first step of sensing a first capacitance across an actuator within the flexible chuck. Next, a step of modifying a voltage across the actuator is performed. Following this, a step of sensing a second capacitance across the actuator is performed. Finally, a step of comparing the first capacitance to the second capacitance is performed, thereby monitoring changes within the flexible chuck.